Field of the Invention
The present invention relates to the technical field of magnetic resonance imaging, and particularly to a magnetic resonance angiography method and apparatus.
Description of the Prior Art
Magnetic resonance imaging (MRI) is an imaging modality using magnetic resonance phenomenon. The principle of magnetic resonance is that the protons of atomic nuclei containing an odd number of protons, such as the nuclei of hydrogen atoms that are widely present in the human body, have spin motion like small magnets, and the spin axes of these small magnets normally have no definite orientation. When an external magnetic field is applied, these small magnets are rearranged according to the lines of magnetic force of the external magnetic field, particularly in the two directions respectively parallel to or anti-parallel to the lines of magnetic force of the external magnetic field. The above-mentioned direction parallel to the line of magnetic force of the external magnetic field is termed as a positive longitudinal axis while the above-mentioned direction anti-parallel to the line of magnetic force of the external magnetic field is termed as a negative longitudinal axis. The atomic nuclei have only longitudinal magnetized vectors, in which the longitudinal magnetized vectors have both direction and amplitude. The atomic nuclei in an external magnetic field are subjected to a pulse excitation using a radio frequency (RF) pulse of a specific frequency, so as to enable the spin axes of these atomic nuclei to deviate from the positive longitudinal axis or the negative longitudinal axis to produce resonance, which is the magnetic resonance phenomenon. The atomic nuclei have transverse magnetized vectors after the spin axes of the above excited atomic nuclei deviate from the positive longitudinal axis or the negative longitudinal axis.
The absorbed energy is gradually released in the form of electromagnetic waves as the excited atomic nuclei emit echo signals after the radio frequency pulse emission stops, then both the phase and energy level of the excited atomic nuclei recover to the states prior to excitation, and an image can be reconstructed after further processing, e.g. spatially encoding the echo signals emitted from the atomic nuclei.
Conventionally, a time-of-flight (TOF) sequence is used in magnetic resonance angiography (MRA) imaging methods. FIG. 1 shows a schematic diagram of a time-of-flight sequence in the prior art. As shown in FIG. 1, the time-of-flight sequence comprises multiple saturation radio frequency pulses SaRFPs and multiple excitation radio frequency pulses ExRFPs, in which one saturation radio frequency pulse SaRFP is followed by one excitation radio frequency pulse ExRFP; the power of the saturation radio frequency pulse SaRFP is much higher than that of the excitation radio frequency pulse ExRFP, furthermore, the duration of the saturation radio frequency pulse SaRFP is much longer than that of the excitation radio frequency pulse ExRFP.
The time-of-flight sequence, before each excitation radio frequency pulse, using saturation radio frequency pulse plus spoiled gradient, saturates all of the spin signals which are parallel to the angiographic plane and located at the region on the side of the distal end, including the venous blood signals within this region. This method can well inhibit the venous blood signals flowing into the angiographic plane during the imaging process. However, the time-of-flight sequence has two disadvantages, one is low imaging efficiency and high time consumption; and the other is high SAR (electromagnetic absorption ratio) caused by intensive circulation of saturation radio frequency pulses SaRFPs.
A magnetic resonance angiography method is comprehensively described in the published patent application no. CN 102749603 A, assigned to Siemens Ltd.